1. Field of the Invention
This invention relates to a method and apparatus for easily and reliably effecting detection of magnetic substances such as metallic pieces which have been erroneously incorporated in various non-magnetic products such as sewn products, woven and/or knitted products, non-woven fabrics, carpets, tatamis (straw matting), foods, and medical products, particularly broken needles which have been erroneously mingled in sewn products in the sewing process.
2. Description of the Prior Art
The possibility that magnetic substances such as metallic pieces will erroneously enter such various products as mentioned above during or after the course of manufacture is undeniable. When the sewing needles attached to sewing machines are broken or the pins are mislaid, they will possibly enter sewn products after the sewing process. When fasteners are sewn to garments, the possibility exists that the leading ends of sewing needles of sewing machines will break off and disappear in the seems between fasteners and garments. The products holding this possibility, therefore, must be deprived of metallic pieces such as lost needles before they are shipped. Now that the liabilities on the part of producers have been growing particularly since the enforcement of the law concerning the products liability, the problem of the entry of broken needles such as is incurred during the course of sewing fasteners to garments has been gaining in seriousness.
Heretofore, the detection of lost needles such as broken needles has been attained by utilizing the fact that a magnetic field is disturbed when a magnetic substance crosses the magnetic field as disclosed in published Japanese Patent Application, KOKAI (Early Publication) No. 4-82,958 and No. 56-36,049, for example. To be specific, when a detection coil is disposed in a magnetic field and a sewn product containing a magnetic substance erroneously incorporated therein is passed through the magnetic field, the magnetic field is disturbed owing to the presence of the magnetic substance and this disturbance of the magnetic field induces a difference in the induced current generated in the detection coil. Since this difference in the induced current is extremely feeble, it is amplified to a prescribed level to permit the detection of the magnetic substance.
In the case of a method for detecting a magnetic substance by utilizing the disturbance of a magnetic field mentioned above, even when a metallic piece which is not a magnetic substance adheres to a sewn product and it happens to have a certain size, this metallic piece never fails to affect the magnetic field and form a cause for mistaken detection. In the case of a fastener to which an aluminum slider or a die-cast zinc slider is attached or a sewn product to which such a fastener is sewn, the fastener or the sewn product, when passing the magnetic field, affects the magnetic field. Then, the apparatus for detecting lost needles fails to discern whether the detected disturbance in the magnetic field (induced current) has been caused by a magnetic substance (needle) mingled in the sewn product or by the slider and forms an erroneous judgement that the sewn product contains a magnetic substance (needle) notwithstanding it is not containing one.
The possible effect of an external noise on the detection cannot be ignored because the difference of the induced current which is generated in the detection coil by the disturbance of the magnetic field is extremely feeble. Since the magnetic substance generates no difference in the induced current flowing through the detection coil unless this magnetic substance is moved, the apparatus is incapable of discriminating whether or not the magnetic substance (needle) is included in a sewn product under test when the sewn product containing the magnetic substance (needle) is transferred at a low speed or it is kept at a stopped state. Conversely, when sewn products are transferred at a high speed, the apparatus incurs difficulty in selecting only the sewn products that are containing magnetic substances (needles).